16 THE HUMAN BODY 



Glycogen or animal starch (CeHioOs) is the anhydride of grape sugar. This 

 is the form in which the excess of sugar is stored in the body to be drawn upon 

 at need. Dextrose is readily converted into it, and it in turn is easily changed 

 back into sugar. In many respects it resembles common vegetable starch. 

 It is present in the muscles of the Body and in the liver, the latter organ alone 

 containing about as much as all the muscles put together. 



Cellulose, the woody fiber of plants, is not found in the Human Body, al- 

 though a chemically identical substance, tunicin, is found in the bodies of 

 tunicates. 



Organic Non-Nitrogenous Acids. Of these the most important 

 is carbon dioxid (COa), which is the form in which by far the greater 

 part of the carbon taken into the Body ultimately leaves it. 

 United with calcium it is found in the bones and teeth in large 

 proportion. 



Formic, acetic, and butyric acids are also found in the Body; stearic, palmitic, 

 and oleic have been above mentioned as obtainable from fats. Lactic acid, 

 CsHeOs, is often present in the digestive tract, and when milk turns sour 

 is formed from lactose. Virtually the same substance, (sarcolactic acid), is 

 formed in muscles when they work or die. 



Glycerin phosphoric acid (CsHgPOe) is obtained on the decomposition of 

 lecithin, a complex nitrogenous fat found in nervous tissue and to some extent 

 in all living cells. 



Physico-Chemical Constitution of the Body. The functioning 

 of the living body is the sum total of the functioning of the in- 

 dividual cells. The activity of any cell, in turn, is determined by 

 the interactions of its constituent molecules. In the living tissues 

 we have a great number of different molecules interacting in ways 

 which depend, in part, on the chemical nature of the molecules, 

 and in part, also, upon the manner in which the molecules are 

 grouped and interrelated physically. The study of the manner in 

 which molecules are related to each other under such conditions 

 as obtain in the Body is a part of the science of Physical Chem- 

 istry and the structure of the Body from this standpoint is its 

 Physico-chemical structure. 



Liquid Environment. Of prime importance from the physico- 

 chemical standpoint is the fact that the active tissues of the body 

 consist largely of water. Molecules in solution in a liquid move 

 about freely, enter and leave chemical combinations readily, and 

 in general display the degree of flexibility essential for the carry- 



